Generally, a rotary machine typified by a centrifugal compressor includes an impeller which is provided on a rotating shaft and a casing which defines a flow path between the impeller and the casing by covering the impeller from the outside. The flow path suctions an external fluid into the casing by rotation of the impeller, applies a pressure to the fluid while the fluid flows through the flow path and discharges the fluid from a casing outlet under a high pressure.
As an example of such a technique, a centrifugal compressor disclosed in the following Patent Literature 1 is known. In the centrifugal compressor disclosed in Patent Literature 1, a so-called closed impeller is adopted. That is, this apparatus includes a rotating shaft which rotates about an axis, a disk which is installed at the rotating shaft, a plurality of blades which are disposed on one side surface of the disk, a closed impeller (impeller) which has a shroud cover provided at an end edge on one side of the plurality of blades in an axial direction, and a casing which forms a flow path by covering the closed impeller from the outside. Due to the above-described constitution, a low-pressure fluid introduced from one side in the axial direction is compressed following the rotation of the impeller and guided to the outside from a discharge portion on a radially outer side as a high-pressure fluid.
However, in the rotary machine described above, a predetermined gap is commonly provided between the impeller which is a rotating body and the casing which is in contact with the impeller in a stationary state. That is, interference between the impeller and the casing is prevented by providing the gap therebetween.